1. Field of the Invention
The present invention relates to a Cu-(copper)-base alloy, and more particularly, to a wear-resistant Cu-base alloy classified as a dispersion strengthened Cu-base alloy, used for an overlay (hardfacing layer) formed on a metal substrate, and having good self-lubricity.
2. Description of the Related Art
Wear-resistant materials of Cu-base alloy include precipitation-hardened alloys such as beryllium (Be) copper alloys and Cu-Ni-Si alloys (e.g., Corson alloy), and dispersion-strengthened alloys in which hard particles of, e.g., oxide (SiO.sub.2, Cr.sub.2 O.sub.3, BeO, TiO.sub.2, ZrO.sub.2, MgO, MnO, etc.) are dispersed in a Cu-base matrix.
Among the Cu-base alloys, the beryllium copper alloy, in particular, has a strength (a tensile strength of 100 kg/mm.sup.2 or more) comparable to that of steel and a high hardness (of HV 300 or more), but when the Cu-base alloys subjected to a precipitation hardening treatment (age hardening treatment) are heated at a temperature (350.degree. to 450.degree. C.) higher than a precipitation (aging) temperature, the hardness thereof is steeply lowered, and thus such an alloy is not suitable as a wear-resistant material. It is not easy to apply the precipitation hardening treatment on large alloy members (parts), and this heat-treatment is apt to generate strain in alloy members (parts) and requires a long treating time. Furthermore, a precipitation is controlled by diffusion in a solid phase, and thus the precipitating particles have a fine size of several micrometers and a relatively high hardness due to the precipitation hardening is attained. Nevertheless, under a wear condition involving sliding (slide abrasion condition), fatal wear often occurs.
One type of the particle dispersion-strengthened Cu-base alloys made through an internal oxidation process has dispersed fine oxide particles similar to the precipitate particles by aging treatment, since such oxide particles are formed by an oxygen diffusion in a solid phase matrix. The precipitation in a solid phase requires a long treating time, and such a heat-treatment is not easily applied to large alloy members and is apt to generate strain. Another type of particle dispersion-strengthened alloys obtained by the sintering process contains dispersed oxide particles having a desired grain size, by controlling a grain size of raw material powders, but it is difficult to attain a uniform dispersion at a micrometer order level. Furthermore, if a Cu-base alloy layer of this type is locally deposited, to form an overlay on a metal substrate, it is necessary to heat the substrate as a whole to a sintering temperature, which generates deformation and strain in the substrate, and thus this type of Cu-base alloy is not suitable for an overlay.
The present inventors have studied particle dispersion-strengthened Cu-base alloys for wear-resistant overlays (hardfacing layers) deposited locally or wholly on a metal substrate, and several of the present inventors have proposed Cu-Ni-Fe-(B)-Si system Cu-base alloys as the wear-resistant dispersion-strengthened Cu-base alloys in which hard particles of silicide and/or boride are dispersed by rapid solidification, in other patent applications. For example, U.S. Pat. No. 4,818,307, based on Japanese Unexamined Patent Publication (Kokai) No. 63-157826, discloses Cu-Ni-Fe-Si-B alloys having dispersed hard particles of silicide and boride of the Fe-Ni system, and Japanese Unexamined Patent Publication (Kokai) No. 1-111831 discloses Cu-Fe-Ni-Cr-Si-(B) alloys having dispersed hard particles of silicide and boride of the Fe-Ni-Cr system.
During the operation of an internal combustion engine (e.g., automobile engine), face portions of exhaust valves of the engine are heated to 700.degree. C. or more, and an exhaust gas has a temperature of 1000.degree. C. or more. Therefore, an overlay of one of the dispersion-strengthened Cu-base alloys deposited on each of valve seats comes into contact with the heated valve face portion, and further, is exposed to the high temperature exhaust gas, and thus the surface temperature of the Cu-base alloy overlay of the valve seat is raised to a high temperature and a portion of the overlay easily adheres to the valve face. Once this adhesion occurs, the Cu-base alloy adhering to the valve comes into contact with the Cu-base alloy overlay, and accordingly, the adhesion becomes much greater and causes considerable wear (adhesion) of the overlay of the valve seat. It has been found that the proposed Cu-base alloys mentioned above, utilizing a strengthening effect of hard particles, cannot prevent adhesion of a Cu-rich phase of a matrix of the Cu-base alloys. It also has been found that conventional dispersion-strengthened Cu-base alloys strengthened by a second phase cannot prevent the adhesion of the Cu-rich phase.
To suppress the adhesion of the Cu-rich phase of a matrix, several of the present inventors proposed a method of dissolving Zn (zinc) and/or Sn (tin) in primary crystals of the matrix (see Japanese Unexamined Patent Publication (Kokai) No. 3-60895), and a method of dispersing Pb (lead) among dentrites of a Cu-base .alpha. phase (see Japanese Unexamined Patent Publication (Kokai) Nos. 1-205043 and 3-87327).
Although the proposed Cu-base alloys improve the resistance to adhesion, the following problems arise. Since the melting points of Zn, Sn and Pb (Zn=420.degree. C., Sn=231.9.degree. C., Pb=327.4.degree. C.) are lower than that of Cu, in a deposition (build-up) process of an overlay of the Cu-base alloy using a laser (as shown in FIG. 1), these alloying elements (metals) are vaporized during the deposition treatment, and thus it is difficult to maintain a desired composition of the alloy. Since the Zn has a high vapor pressure, a plasma is easily generated, and thus a bead (i.e., deposited layer) is not stably formed. Furthermore, the Pb vapor is toxic, and thus a safety system, such as an exhaust apparatus is required.